Field of the Invention
The present invention is directed to bi-specific monovalent diabodies that comprise an immunoglobulin Fc Domain (“bi-specific monovalent Fc diabodies”) and are composed of three polypeptide chains and which possess at least one binding site specific for an epitope of CD32B and one binding site specific for an epitope of CD79b (i.e., a “CD32B×CD79b Fc diabody”). The bi-specific monovalent Fc diabodies of the present invention are capable of simultaneous binding to CD32B and CD79b. The invention is directed to such compositions, to pharmaceutical compositions that contain such bi-specific monovalent Fc diabodies and to methods for their use in the treatment of inflammatory diseases or conditions, and in particular, systemic lupus erythematosus (SLE) and graft vs. host disease.
Description of Related Art
I. The Fcγ Receptors and CD32B
The interaction of antibody-antigen complexes with cells of the immune system results in a wide array of responses, ranging from effector functions such as antibody-dependent cytotoxicity, mast cell degranulation, and phagocytosis to immunomodulatory signals such as regulating lymphocyte proliferation and antibody secretion. All these interactions are initiated through the binding of the Fc Domain of antibodies or immune complexes to specialized cell-surface receptors on hematopoietic cells. The diversity of cellular responses triggered by antibodies and immune complexes results from the structural heterogeneity of Fc receptors. Fc receptors share structurally related ligand binding domains which presumably mediate intracellular signaling.
The Fc receptors are members of the immunoglobulin gene superfamily of proteins. They are surface glycoproteins that can bind the Fc portion of immunoglobulin molecules. Each member of the family recognizes immunoglobulins of one or more isotypes through a recognition domain on the a chain of the Fc receptor.
Fc receptors are defined by their specificity for immunoglobulin subtypes (see, Ravetch J. V. et al. (1991) “Fc Receptors,” Annu Rev. Immunol. 9:457-92; Gerber J. S. et al. (2001) “Stimulatory And Inhibitory Signals Originating From The Macrophage Fcγ Receptors,” Microbes and Infection, 3:131-139; Billadeau D. D. et al. (2002) “ITAMs Versus ITIMs: Striking A Balance During Cell Regulation,” J. Clin. Invest. 2(109):161-1681; Ravetch J. V. et al. (2000) “Immune Inhibitory Receptors,” Science 290:84-89; Ravetch J. V. et al. (2001) “IgG Fc Receptors,” Annu. Rev. Immunol. 19:275-90; Ravetch J. V. (1994) “Fc Receptors: Rubor Redux,” Cell, 78(4): 553-60).
Fc receptors that are capable of binding to IgG antibodies are termed “FcγRs.” Each member of this family is an integral membrane glycoprotein, possessing extracellular domains related to a C2-set of immunoglobulin-related domains, a single membrane spanning domain and an intracytoplasmic domain of variable length. There are three known FcγRs, designated FcγRI(CD64), FcγRII(CD32), and FcγRIII(CD16). The three receptors are encoded by distinct genes; however, the extensive homologies between the three family members suggest they arose from a common progenitor perhaps by gene duplication.
FcγRII(CD32) proteins are 40 KDa integral membrane glycoproteins which bind only the complexed IgG due to a low affinity for monomeric Ig (106 M−1). This receptor is the most widely expressed FcγR, present on all hematopoietic cells, including monocytes, macrophages, B cells, NK cells, neutrophils, mast cells, and platelets. FcγRII has only two immunoglobulin-like regions in its immunoglobulin binding chain and hence a much lower affinity for IgG than FcγRI. There are three human FcγRII genes (FcγRIIA(CD32A), FcγRIIB(CD32B), FcγRIIC(CD32C)), all of which bind IgG in aggregates or immune complexes.
Distinct differences within the cytoplasmic domains of the FcγRIIA and FcγRIIB create two functionally heterogenous responses to receptor ligation. The fundamental difference is that, upon binding to an IgG Fc region, the FcγRIIA isoform initiates intracellular signaling leading to immune system activation (e.g., phagocytosis, respiratory burst, etc.), whereas, upon binding to an IgG Fc region, the FcγRIIB isoform initiates signals that lead to the dampening or inhibition of the immune system (e.g., inhibiting B cell activation, etc.).
Such activating and inhibitory signals are both transduced through the FcγRs following ligation to an IgG Fc region. These diametrically opposing functions result from structural differences among the different receptor isoforms. Two distinct domains within the cytoplasmic signaling domains of the receptor called Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) or Immunoreceptor Tyrosine-Based Inhibitory Motifs (ITIMS) account for the different responses. The recruitment of different cytoplasmic enzymes to these structures dictates the outcome of the FcγR-mediated cellular responses. ITAM-containing FcγR complexes include FcγRI, FcγRIIA, FcγRIIIA, whereas ITIM-containing complexes only include FcγRIIB.
Human neutrophils express the FcγRIIA gene. FcγRIIA clustering via immune complexes or specific antibody cross-linking serves to aggregate ITAMs along with receptor-associated kinases which facilitate ITAM phosphorylation. ITAM phosphorylation serves as a docking site for Syk kinase, activation of which results in activation of downstream substrates (e.g., PI3K). Cellular activation leads to release of pro-inflammatory mediators.
The FcγRIIB gene is expressed on B lymphocytes; its extracellular domain is 96% identical to FcγRIIA and binds IgG complexes in an indistinguishable manner. The presence of an ITIM in the cytoplasmic domain of FcγRIIB defines this inhibitory subclass of FcγR. The molecular basis of this inhibition has been established. When FcγRIIB becomes co-ligated to an activating receptor by way of the Fc regions of the IgG immunoglobulins of an immune complex, the FcγRIIB ITIM becomes phosphorylated and attracts the SH2 domain of the inositol polyphosphate 5′-phosphatase (SHIP), which hydrolyzes phosphoinositol messengers released as a consequence of ITAM-containing, FcγR-mediated tyrosine kinase activation, consequently preventing the influx of intracellular Ca++. Thus such cross-linking of FcγRIIB and an activating receptor dampens the activity of the activating receptor, and thus inhibits cellular responsiveness. Thus, on B-cells, B-cell activation, B-cell proliferation and antibody secretion is dampened or aborted. Thus, at the onset of antigen detection, monomeric IgG-antigen bonding occurs, and the Fc regions of bound antibodies bind to ITAMs of the activating FcγRs to mediate activation of the immune system. As the host's response progresses, multimeric IgG-antigen immune complexes form that are capable of binding to FcγRIIB (thus co-ligating such complexes with an activating receptor), leading to the dampening and ultimate cessation of the immune response (see, e.g., U.S. Pat. Nos. 8,445,645; 8,217,147; 8,216,579; 8,216,574; 8,193,318; 8,192,737; 8,187,593; 8,133,982; 8,044,180; 8,003,774; 7,960,512; 7,786,270; 7,632,497; 7,521,542; 7,425,619; 7,355,008 and United States Patent Publications No.: 2012/0276094; 2012/0269811; 2012/0263711; 2012/0219551; 2012/0213781; 2012/0141476; 2011/0305714; 2011/0243941; 2010/0322924; 2010/0254985; 2010/0196362; 2010/0174053; 2009/0202537; 2009/0191195; 2009/0092610; 2009/0076251; 2009/0074771; 2009/0060910; 2009/0053218; 2009/0017027; 2009/0017026; 2009/0017023; 2008/0138349; 2008/0138344; 2008/0131435; 2008/0112961; 2008/0044429; 2008/0044417; 2007/0077246; 2007/0036799; 2007/0014795; 2007/0004909; 2005/0260213; 2005/0215767; 2005/0064514; 2005/0037000; 2004/0185045).
II. The B-Cell Receptor and CD79b
B cells are immune system cells that are responsible for producing antibodies. The B-cell response to antigen is an essential component of the normal immune system. B-cells possess specialized cell-surface receptors (B-cell receptors; “BCR”). If a B-cell encounters an antigen capable of binding to that cell's BCR, the B-cell will be stimulated to proliferate and produce antibodies specific for the bound antigen. To generate an efficient response to antigens, BCR-associated proteins and T-cell assistance are also required. The antigen/BCR complex is internalized, and the antigen is proteolytically processed. A small part of the antigen remains complexed with major histocompatability complex-II (“MHC-II”) molecules on the surface of the B cells where the complex can be recognized by T-cells. T-cells activated by such antigen presentation secrete a variety of lymphokines that induce B-cell maturation.
Signaling through the BCR plays an important role in the generation of antibodies, in autoimmunity, and in the establishment of immunological tolerance (Gauld, S. B. et al. (2002) “B Cell Antigen Receptor Signaling: Roles In Cell Development And Disease,” Science 296(5573):1641-1642). Immature B cells that bind self-antigens while still in the bone marrow are eliminated by apoptosis. In contrast, antigen binding on mature B cells results in activation, proliferation, anergy and apoptosis. The particular functional response observed depends upon whether the B-cell receives co-stimulatory signals through other surface receptors and the specific signal transduction pathways that are activated.
The BCR is composed of a membrane immunoglobulin which, together with non-covalently associated α and β subunits of CD79 (“CD79a” and “CD79b,” respectively), forms the BCR complex. CD79a and CD79b are signal transducing subunits that contain a conserved immunoreceptor tyrosine-based activation motif (“ITAM”) required for signal transduction (Dylke, J. et al. (2007) “Role of the extracellular and transmembrane domain of Ig-alpha/beta in assembly of the B cell antigen receptor (BCR),” Immunol. Lett. 112(1):47-57; Cambier, J. C. (1995) “New Nomenclature For The Reth Motif (or ARH1/TAM/ARAM/YXXL),” Immunol. Today 16:110). Aggregation of the BCR complex by multivalent antigen initiates transphosphorylation of the CD79a and CD79b ITAMs and activation of receptor-associated kinases (DeFranco, A. L. (1997) “The Complexity Of Signaling Pathways Activated By The BCR,” Curr. Opin. Immunol. 9:296-308; Kurosaki, T. (1997) “Molecular Mechanisms In B-Cell Antigen Receptor Signaling,” Curr. Opin. Immunol. 9:309-318; Kim, K. M. et al. (1993) “Signalling Function Of The B-Cell Antigen Receptors,” Immun. Rev. 132:125-146). Phosphorylated ITAMs recruit additional effectors such as PI3K, PLC-γ and members of the Ras/MAPK pathway. These signaling events are responsible for both the B cell proliferation and increased expression of activation markers (such as MHC-II and CD86) that are required to prime B cells for their subsequent interactions with T-helper (“Th”) cells.
III. Inflammatory Diseases or Conditions
Inflammation is a process by which the body's white blood cells and chemicals protect our bodies from infection by foreign substances, such as bacteria and viruses. It is usually characterized by pain, swelling, warmth and redness of the affected area. Chemicals known as cytokines and prostaglandins control this process, and are released in an ordered and self-limiting cascade into the blood or affected tissues. This release of chemicals increases the blood flow to the area of injury or infection, and may result in the redness and warmth. Some of the chemicals cause a leak of fluid into the tissues, resulting in swelling. This protective process may stimulate nerves and cause pain. These changes, when occurring for a limited period in the relevant area, work to the benefit of the body.
Inflammatory diseases or conditions reflect an immune system attack on a body's own cells and tissue (i.e., an “autoimmune” response). There are many different autoimmune disorders which affect the body in different ways. For example, the brain is affected in individuals with multiple sclerosis, the gut is affected in individuals with Crohn's disease, and the synovium, bone and cartilage of various joints are affected in individuals with rheumatoid arthritis. As autoimmune disorders progress destruction of one or more types of body tissues, abnormal growth of an organ, or changes in organ function may result. The autoimmune disorder may affect only one organ or tissue type or may affect multiple organs and tissues. Organs and tissues commonly affected by autoimmune disorders include red blood cells, blood vessels, connective tissues, endocrine glands (e.g., the thyroid or pancreas), muscles, joints, and skin. Examples of autoimmune disorders include, but are not limited to, Hashimoto's thyroiditis, pernicious anemia, Addison's disease, type 1 diabetes, rheumatoid arthritis, systemic lupus erythematosus (SLE), dermatomyositis, Sjogren's syndrome, dermatomyositis, lupus erythematosus, multiple sclerosis, autoimmune inner ear disease myasthenia gravis, Reiter's syndrome, Graves' disease, autoimmune hepatitis, familial adenomatous polyposis and ulcerative colitis.
Inflammatory diseases or conditions can also arise when the body's normally protective immune system causes damage by attacking foreign cells or tissues whose presence is beneficial to the body (e.g., the rejection of transplants (host vs. host disease)) or from the rejection of the cells of an immunosuppressed host by immunocompetent cells of an introduced transplant graft (graft vs. host disease) (DePaoli, A. M. et al. (1992) “Graft-Versus-Host Disease And Liver Transplantation,” Ann. Intern. Med. 117:170-171; Sudhindran, S. et al. (2003) “Treatment Of Graft-Versus-Host Disease After Liver Transplantation With Basiliximab Followed By Bowel Resection,” Am J Transplant. 3:1024-1029; Pollack, M. S. et al. (2005) “Severe, Late-Onset Graft-Versus-Host Disease In A Liver Transplant Recipient Documented By Chimerism Analysis,” Hum. Immunol. 66:28-31; Perri, R. et al. (2007) “Graft Vs. Host Disease After Liver Transplantation: A New Approach Is Needed,” Liver Transpl. 13:1092-1099; Mawad, R. et al. (2009) “Graft-Versus-Host Disease Presenting With Pancytopenia After En Bloc Multiorgan Transplantation: Case Report And Literature Review,” Transplant Proc. 41:4431-4433; Akbulut, S. et al. (2012) “Graft-Versus-Host Disease After Liver Transplantation: A Comprehensive Literature Review,” World J. Gastroenterol. 18(37): 5240-5248).
Despite recent advances in the treatment of such diseases or conditions, a need continues to exist for compositions capable of treating or preventing inflammatory diseases or conditions.
IV. Bi-Specific Diabodies
The ability of an intact, unmodified antibody (e.g., an IgG) to bind an epitope of an antigen depends upon the presence of variable domains on the immunoglobulin light and heavy chains (i.e., the VL and VH Domains, respectively). The design of a diabody is based on the single chain Fv construct (scFv) (see, e.g., Holliger et al. (1993) “‘Diabodies’: Small Bivalent And Bispecific Antibody Fragments,” Proc. Natl. Acad. Sci. (U.S.A.) 90:6444-6448; US 2004/0058400 (Hollinger et al.); US 2004/0220388 (Mertens et al.); Alt et al. (1999) FEBS Lett. 454(1-2):90-94; Lu, D. et al. (2005) “A Fully Human Recombinant IgG-Like Bispecific Antibody To Both The Epidermal Growth Factor Receptor And The Insulin-Like Growth Factor Receptor For Enhanced Antitumor Activity,” J. Biol. Chem. 280(20):19665-19672; WO 02/02781 (Mertens et al.); Olafsen, T. et al. (2004) “Covalent Disulfide-Linked Anti-CEA Diabody Allows Site-Specific Conjugation And Radiolabeling For Tumor Targeting Applications,” Protein Eng Des Sel. 17(1):21-27; Wu, A. et al. (2001) “Multimerization Of A Chimeric Anti-CD20 Single Chain Fv-Fv Fusion Protein Is Mediated Through Variable Domain Exchange,” Protein Engineering 14(2):1025-1033; Asano et al. (2004) “A Diabody For Cancer Immunotherapy And Its Functional Enhancement By Fusion Of Human Fc Region,” Abstract 3P-683, J. Biochem. 76(8):992; Takemura, S. et al. (2000) “Construction Of A Diabody (Small Recombinant Bispecific Antibody) Using A Refolding System,” Protein Eng. 13(8):583-588; Baeuerle, P. A. et al. (2009) “Bispecific T-Cell Engaging Antibodies For Cancer Therapy,” Cancer Res. 69(12):4941-4944).
Interaction of an antibody light chain and an antibody heavy chain and, in particular, interaction of its VL and VH Domains forms one of the epitope binding sites of the antibody. In contrast, the scFv construct comprises VL and VH Domains of an antibody contained in a single polypeptide chain wherein the domains are separated by a flexible linker of sufficient length to allow self-assembly of the two domains into a functional epitope binding site. Where self-assembly of the VL and VH Domains is rendered impossible due to a linker of insufficient length (less than about 12 amino acid residues), two of the scFv constructs interact with one another other to form a bivalent molecule in which the VL Domain of one chain associates with the VH Domain of the other (reviewed in Marvin et al. (2005) “Recombinant Approaches To IgG-Like Bispecific Antibodies,” Acta Pharmacol. Sin. 26:649-658).
Natural antibodies are capable of binding to only one epitope species (i.e., mono-specific), although they can bind multiple copies of that species (i.e., exhibiting bi-valency or multi-valency). The art has noted the capability to produce diabodies that differ from such natural antibodies in being capable of binding two or more different epitope species (i.e., exhibiting bi-specificity or multispecificity in addition to bi-valency or multi-valency) (see, e.g., Holliger et al. (1993) “‘Diabodies’: Small Bivalent And Bispecific Antibody Fragments,” Proc. Natl. Acad. Sci. (U.S.A.) 90:6444-6448; US 2004/0058400 (Hollinger et al.); US 2004/0220388 (Mertens et al.); Alt et al. (1999) FEBS Lett. 454(1-2):90-94; Lu, D. et al. (2005) “A Fully Human Recombinant IgG-Like Bispecific Antibody To Both The Epidermal Growth Factor Receptor And The Insulin-Like Growth Factor Receptor For Enhanced Antitumor Activity,” J. Biol. Chem. 280(20):19665-19672; WO 02/02781 (Mertens et al.); Mertens, N. et al., “New Recombinant Bi-and Trispecific Antibody Derivatives,” In: NOVEL FRONTIERS IN THE PRODUCTION OF COMPOUNDS FOR BIOMEDICAL USE, A. VanBroekhoven et al. (Eds.), Kluwer Academic Publishers, Dordrecht, The Netherlands (2001), pages 195-208; Wu, A. et al. (2001) “Multimerization Of A Chimeric Anti-CD20 Single Chain Fv-Fv Fusion Protein Is Mediated Through Variable Domain Exchange,” Protein Engineering 14(2): 1025-1033; Asano et al. (2004) “A Diabody For Cancer Immunotherapy And Its Functional Enhancement By Fusion Of Human Fc Region,” Abstract 3P-683, J. Biochem. 76(8):992; Takemura, S. et al. (2000) “Construction Of A Diabody (Small Recombinant Bispecific Antibody) Using A Refolding System,” Protein Eng. 13(8):583-588; Baeuerle, P. A. et al. (2009) “Bispecific T-Cell Engaging Antibodies For Cancer Therapy,” Cancer Res. 69(12):4941-4944).
The provision of non-monospecific diabodies provides a significant advantage: the capacity to co-ligate and co-localize cells that express different epitopes. Bivalent diabodies thus have wide-ranging applications including therapy and immunodiagnosis. Bi-valency allows for great flexibility in the design and engineering of the diabody in various applications, providing enhanced avidity to multimeric antigens, the cross-linking of differing antigens, and directed targeting to specific cell types relying on the presence of both target antigens. Due to their increased valency, low dissociation rates and rapid clearance from the circulation (for diabodies of small size, at or below ˜50 kDa), diabody molecules known in the art have also shown particular use in the field of tumor imaging (Fitzgerald et al. (1997) “Improved Tumour Targeting By Disulphide Stabilized Diabodies Expressed In Pichia pastoris,” Protein Eng. 10:1221). Of particular importance is the co-ligating of differing cells, for example, the cross-linking of cytotoxic T-cells to tumor cells (Staerz et al. (1985) “Hybrid Antibodies Can Target Sites For Attack By T Cells,” Nature 314:628-631, and Holliger et al. (1996) “Specific Killing Of Lymphoma Cells By Cytotoxic T-Cells Mediated By A Bispecific Diabody,” Protein Eng. 9:299-305).
Diabody epitope binding domains may also be directed to a surface determinant of any immune effector cell such as CD3, CD16, CD32, or CD64, which are expressed on T lymphocytes, natural killer (NK) cells or other mononuclear cells. In many studies, diabody binding to effector cell determinants, e.g., Fcγ receptors (FcγR), was also found to activate the effector cell (Holliger et al. (1996) “Specific Killing Of Lymphoma Cells By Cytotoxic T-Cells Mediated By A Bispecific Diabody,” Protein Eng. 9:299-305; Holliger et al. (1999) “Carcinoembryonic Antigen (CEA)-Specific T-cell Activation In Colon Carcinoma Induced By Anti-CD3×Anti-CEA Bispecific Diabodies And B7×Anti-CEA Bispecific Fusion Proteins,” Cancer Res. 59:2909-2916; WO 2006/113665; WO 2008/157379; WO 2010/080538; WO 2012/018687; WO 2012/162068). Normally, effector cell activation is triggered by the binding of an antigen bound antibody to an effector cell via Fc-FcγR interaction; thus, in this regard, diabody molecules of the invention may exhibit Ig-like functionality independent of whether they comprise an Fc Domain (e.g., as assayed in any effector function assay known in the art or exemplified herein (e.g., ADCC assay)). By cross-linking tumor and effector cells, the diabody not only brings the effector cell within the proximity of the tumor cells but leads to effective tumor killing (see e.g., Cao et al. (2003) “Bispecific Antibody Conjugates In Therapeutics,” Adv. Drug. Deliv. Rev. 55:171-197).
However, the above advantages come at salient cost. The formation of such non-monospecific diabodies requires the successful assembly of two or more distinct and different polypeptides (i.e., such formation requires that the diabodies be formed through the heterodimerization of different polypeptide chain species). This fact is in contrast to mono-specific diabodies, which are formed through the homodimerization of identical polypeptide chains. Because at least two dissimilar polypeptides (i.e., two polypeptide species) must be provided in order to form a non-monospecific diabody, and because homodimerization of such polypeptides leads to inactive molecules (Takemura, S. et al. (2000) “Construction Of A Diabody (Small Recombinant Bispecific Antibody) Using A Refolding System,” Protein Eng. 13(8):583-588), the production of such polypeptides must be accomplished in such a way as to prevent covalent bonding between polypeptides of the same species (Takemura, S. et al. (2000) “Construction Of A Diabody (Small Recombinant Bispecific Antibody) Using A Refolding System,” Protein Eng. 13(8):583-588). The art has therefore taught the non-covalent association of such polypeptides (see, e.g., Olafsen et al. (2004) “Covalent Disulfide-Linked Anti-CEA Diabody Allows Site-Specific Conjugation And Radiolabeling For Tumor Targeting Applications,” Prot. Engr. Des. Sel. 17:21-27; Asano et al. (2004) “A Diabody For Cancer Immunotherapy And Its Functional Enhancement By Fusion Of Human Fc Region,” Abstract 3P-683, J. Biochem. 76(8):992; Takemura, S. et al. (2000) “Construction Of A Diabody (Small Recombinant Bispecific Antibody) Using A Refolding System,” Protein Eng. 13(8):583-588; Lu, D. et al. (2005) “A Fully Human Recombinant IgG-Like Bispecific Antibody To Both The Epidermal Growth Factor Receptor And The Insulin-Like Growth Factor Receptor For Enhanced Antitumor Activity,” J. Biol. Chem. 280(20): 19665-19672).
However, the art has recognized that bi-specific diabodies composed of non-covalently associated polypeptides are unstable and readily dissociate into non-functional monomers (see, e.g., Lu, D. et al. (2005) “A Fully Human Recombinant IgG-Like Bispecific Antibody To Both The Epidermal Growth Factor Receptor And The Insulin-Like Growth Factor Receptor For Enhanced Antitumor Activity,” J. Biol. Chem. 280(20):19665-19672).
In the face of this challenge, the art has succeeded in developing stable, covalently bonded heterodimeric non-monospecific diabodies (see, e.g., WO 2006/113665; WO/2008/157379; WO 2010/080538; WO 2012/018687; WO/2012/162068; Johnson, S. et al. (2010) “Effector Cell Recruitment With Novel Fv-Based Dual-Affinity Re-Targeting Protein Leads To Potent Tumor Cytolysis And In Vivo B-Cell Depletion,” J. Molec. Biol. 399(3):436-449; Veri, M. C. et al. (2010) “Therapeutic Control Of B Cell Activation Via Recruitment Of Fcgamma Receptor IIb (CD32B) Inhibitory Function With A Novel Bispecific Antibody Scaffold,” Arthritis Rheum. 62(7):1933-1943; Moore, P. A. et al. (2011) “Application Of Dual Affinity Retargeting Molecules To Achieve Optimal Redirected T-Cell Killing Of B-Cell Lymphoma,” Blood 117(17):4542-4551). Such approaches involve engineering one or more cysteine residues into each of the employed polypeptide species. For example, the addition of a cysteine residue to the C-terminus of such constructs has been shown to allow disulfide bonding between the polypeptide chains, stabilizing the resulting heterodimer without interfering with the binding characteristics of the bivalent molecule.
Notwithstanding such success, the production of stable, functional heterodimeric, non-monospecific can be further improved by the careful consideration and placement of the domains employed in the polypeptide chains. The present invention is thus directed to the provision of specific polypeptides that are particularly designed to form, via covalent bonding, heterodimeric Fc diabodies that are capable of simultaneously binding CD32B and CD79b.